NBSIR 77-1202

Application of Proof Tests to

Silicon Nitride

N. J. Tighe, S. M. Wiederhorn and L. R. Russell

Inorganic Materials Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

Preprint "Failure Criteria for Ceramics”, ASME Gas Turbine Conference,

Philadelphia, Pennsylvania

March 27 - 31, 1977

Prepared for United States Air Force

Air Force Materials Laboratory (AFSC)

Aeronautical Systems Division

Wright Patterson AFB, Ohio 45433

i9m

NBSIR 77-1202

APPLICATION OF PROOF TESTS TO

SILICON NITRIDE

N. J. Tighe, S. M. Wiederhorn and L. R. Russell

Inorganic Materials Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

Preprint "Failure Criteria for Ceramics", ASME Gas Turbine Conference,

Philadelphia, Pennsylvania

March 27 - 31, 1977

Prepared for United States Air Force

Air Force Materials Laboratory (AFSC)

Aeronautical Systems Division

Wright Patterson AFB, Ohio 45433

U.S. DEPARTMENT OF COMMERCE. Juanita M. Kreps, Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director
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APPLICATION OF PROOF TESTS TO SILICON NITRIDE

N. J. Tighe, S. M. Wiederhorn, L. R. Russell

Inorganic Materials Division

National Bureau of Standards

ABSTRACT

In the ceramic turbine program, the high cost of turbine components and

the disastrous effect of component failure require development of both

destructive and nondestructive tests for detecting and rejecting defective

components. Proof tests, in which a load is applied to break weak

components, will ensure that the survivors will have a probability of

failure that is acceptable for the design requirements. Thus, the

development of a reliable proof test program requires testing a statistically

significant number of specimens. Proof tests have been applied to

assure the reliability of glass components and are being developed for

polycrystalline ceramic components, such as silicon nitride. The initial

proof tests are being done with 4 point bend specimens of hot-pressed

and reaction-bonded silicon nitride. The tests are conducted at room

temperature and at the maximum useful temperature of the materials.

1. Introduction

In order to assure the reliability of ceramic components for turbine

engines it is necessary to use an evaluation program which will eliminate

flawed and low strength components. Failure prediction techniques based

on fracture mechanics data are being developed and evaluated for this

testing The present paper describes results obtained as part of a

program to evaluate the proof testing technique. In proof testing, weak

components are eliminated by applying a load larger than the expected

service load. The survivors of this test then should have predictable
lifetimes in service. This method was evaluated for glass components by

Wiederhorn^, and for silicon carbide tested at 800 and 1400°C by Evans

and Lange

The validity of the proof testing procedure must be established on

laboratory specimens by testing significant numbers of specimens under

conditions similar to service conditions. If the method is reliable

then it can be extended to components. The proof test test method has

not been evaluated for silicon nitride. In this paper we present results

obtained on hot-pressed, magnesia-doped silicon nitride by proof-testing

at 25°C and at 1200°C. Above 1200°C' the material deforms plastically

and has short predicted lifetimes.

2. Experimental Procedure

2.1 Materials

Specimens were made from two billets of hot-pressed magnesia-doped

silicon nitride. These billets were manufactured at different times but

had the same nominal composition. The typical impurities (wt %) are:

0.20A1, 0.05Ca, 0.40-0.64Fe, 0.70-0.74Mg, 2.6-2.7W. Tungsten particles

were picked-up during powder preparation.

2.2 Test Procedure

Test bars 4x5x50 mm were cut from the billets and were ground along

their length with a 180 grit diamond wheel. From the billet designated-A

120 bars were cut at different times. Specimens for crack propagation

tests were cut from the remainder of the billet. Three hundred bars

were cut from billet-B. These bars were numbered and were randomized

for testing.

2
The bars were tested in 4-pt bending using a silicon carbide jig

with upper and lower spans of 10 and 40 mm respectively. The proof test

procedure was to load the specimen at constant stress rate (crosshead

speed of 0.2mm/m) until the calculated proof stress was reached; and,

then, to unload at the same rate. The specimens that survived the proof

tests were left in the jig and were broken using the same loading conditions.

The test temperatures were 25°C and 1200°C. For the high temperature

tests the furnace was heated at 'v^20°/min and the specimens and test rig

were equilibrated for 1/2 hr at 1200°C before loading. Additional tests

at times of 2, 16, 64 and 100 hrs were made to determine the effects of

oxidation and of heating on the strength of the material.

The proof stress was chosen from the flexure strength data to break

35 to 50% of the specimens.

3. Results of Proof Testing

The proof test results are presented as Welbull plots of the strength

distribution. At room temperature the application of a proof stress of

630 MPa broke ^^^50% of the specimens. The Weibull plot in Fig. 1 shows

the initial distribution (inert strength) and shows that the proof test

survivors all broke above the proof stress. Thus the strength distribution

is clearly truncated by a room temperature proof test. Similar results

were obtained with specimens from billet A. Fracture initiated primarily

at the surface.

In order to test the effectiveness of the proof test procedure for

high temperature use, another group of 80 specimens was used to obtain

the data in Fig. 2. The data in this plot were obtained by applying

3
Fig. 1 Effect of proof testing on

the strength distribution of

magnesia-doped hot pressed silicon

nitride, billet-B, proof stress

630 MN m~2.

Fig. 2 Effect of proof testing at

1200°C on the strength distribution,

billet-B, proof stress 500 MN m“^

.

proof stress of 500 MPa which was calculated to break 35% of the specimens

at 1200°C. The strength distribution of the survivors shows some truncation

and a slight increase in average strength. However 2 specimens broke

below the proof stress after proof testing. The difference between the

25° and 1200°C results suggests that the flaw distribution which limits

strength is different at the two temperatures. The specimens were held

at 1200°C for 1/2 hour before applying the proof test. It may be that

both oxidation and annealing affected surface flaws.

Since some degradation of strength can occur as a result of oxidation,

the survivors of the 25°C proof test (630 MPa) were heated for 1/2 hr at

1200°C, cooled to 25°C and broken. The results of this test are shown

in Fig. 3. It is seen that the strength distribution has changed but

i;
2S0 3&0 450 550 650 750 050 950

FLEXURE STREM6TH. MNm~^

Fig. 3 Effect of oxidation on strength distribution, proof

test survivors were heated 1/2

hr., at 1200°C, cooled and

broken at 25°C, billet-B.

FLEXURE STRENGTH, MNm^

Fig. 4 Effect of proof testing on strength distribution, survivors of

25°C proof test were broken at

1200°C, proof test 650 MN m“^.

^50% of the survivors broke below the proof load of 630 Mpa. Comparing

these results with those in Fig. 1, it is seen that a substantial change

occurs in the strength distribution of the proof test survivors after

1/2 hour oxidation.

The data shown in Fig. 4 were obtained with specimens from billet

j A. These specimens were proof tested at 25°C and the survivors were

i

i broken at 1200°C. The strength distribution is not changed much between

. the proof test survivors and the initial distribution.

i!

I The series of 80 specimens oxidized for 1/2, 2, 16, 64 and 100

i\

|j hrs., has not been completed. Initial results show a slight

: increase in strength after 1/2, 2, and 16 hrs., and a decrease after 64

and 100 hrs.

5
4. Discussion of the Proof Test Results

The results presented here demonstrate that the strength distribution

at 1200°C is sufficiently different from the inert strength that some of

the proof testing assumptions must be reassessed. The flaws that limit

the flexure strength at room temperature appear to comprise a different

population from the flaws that limit the strength at 1200°C. Where as,

in the development of the proof test theory it was assumed that the flaw

population did not change.

It is clear from our test program that considerable oxidation

occurs at 1200°C. In our preliminary- tests there is a slight change in

the strength distribution and an improvement in strength but by the time

the specimen has been heated 64-100 hrs., the strength distribution is

again close to the initial distribution. The oxidation can be detrimental

to the room temperature strength, because the oxides that form, namely

cristobalite, enstatite, silicon oxynitride and glass^ have a larger

specific volume than the silicon nitride. These oxides fracture on

cooling and the oxide film changes composition during longtime heating.

The oxidation or annealing can be beneficial to the strength by healing

the strength limiting flaws or by relieving the strain around such

f laws

If the proof test is required to eliminate weak specimens from the

components that will be used at temperatures as high as 1200‘’C, then,

these preliminary results suggest that specimens will have to be proof

tested near their service temperatures. Clearly this is a difficult

process for some components.

6
The results we have obtained are based on statistically significant

sample sizes. We found that data from fewer than 10 specimens did not

give a reliable distribution curve. The randomization of the specimens

from billet B ensured that the distributions were free of bias related

to preparation method or to inhomogenities in the billets.

5. Conclusions

The results demonstrate that proof testing is valid for hot-pressed

silicon nitride when the testing is done at room temperature i.e. under

inert conditions. When the testing is carried out at 1200°C, i.e. under

corrosive conditions, the proof test approach to assuring reliability

may not be valid. The strength distribution and hence the flaw distribution

are changed by the heating and oxidation during the tests and the strength

distribution was changed only slightly by the proof tests.

Acknowledgment

This work was supported by the USAF under grant no. F3361576F6752

.

7
References

1. Evans, A. G., and Wlederhorn, S.M., "Crack Propagation and Failure

Prediction in Silicon Nitride at Elevated Temperatures," J. Matl.

Sci. £, 270-278 (1974).

2. Ritter, J. E. , Jr., and Meisel, J. A. ,_ "Strength and Failure Prediction

for Glass and Ceramics", J. Am. Ceram. Soc. 59 , 1112 (1976).

3. Evans, A. G. , and Wlederhorn, S. M. , "Proof Testing of Ceramic

Materials - An Analytical Basis for Failure Prediction". Int. J.

Fracture, 1^ 379-392 (1974)

4. Wlederhorn, S.M., "Prevention of Failure in Glass by Proof-Testing,"

J. Am. Ceram. Soc. 5^ 227-8 (1973).

5. Evans, A. G., and Lange, F. F. , ^"Crack Propagation and Failure in

Glass by Proof testing," J. Mater. Sci. ^ 1659-1664 (1975).

6. Wlederhorn, S. M. , Fuller, E. R. , Jr., and Mandel, J., "An Error

Analysis of Failure Prediction Techniques Derived from Fracture

Mechanics," J. Am. Ceram. Soc. ^ 403-411 (1976).

7. Tighe, N. J., "Microstructure of Oxidized Silicon Nitride", Proc.

EMSA, Claitors Publ., New Orleans, p 420-421, 1974.

8
NBS-1 14A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUHI.K A VION OK Ki:iK)K V NO. NBSIR 77-1202

2. Gov't Accession

No.

3. Kc( ipicni‘s Atccs-sion N(;.

4. TITl.l'; AND SUBTITUi

Application of Proof Tests to Silicon Nitride

5. Fubl ic at ion Date March 1977

6. Performing Organizaiion Code

7. AUTHOR(S)

N. J. Tighe, S. M. Wiederhorn and L. R. Russell

8. Performing C.)rgnn. I^eport No.

9, PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Pro jeci /Task/Work Unit No.

11. Contract/Grant No.

F33615-76-F-6752

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

United States Air Force

Air Force Materials Laboratory (AFSC)

Aeronautical Systems Division

Wright Patterson AFB. Ohio 4543.1

13. Type of Report & Period Covered

14. Sponsoring Agency C.ode

15. SUPPLEMENTARY NOTES

16. ABSTRACT (VI 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) In the ceramic turbine program, the high cost of turbine components and the

disastrous effect of component failure require development of both destructive and

nondestructive tests for detecting and rejecting defective components. Proof tests, in

which a load is applied to break weak components, will ensure that the survivors will

have a possibility of failure that is acceptable for the design requirements. Thus,

the development of a reliable proof test program requires testing a statistically

significant number of specimens. Proof tests have been applied to assure the reliability of glass components and are being developed for polycrystalline ceramic components, such as silicon nitride. The initial proof tests are being done with 4 Pt bend

specimens of hot-pressed and reaction-bonded silicon nitride. The tests are conducted

at room temperature and at the maximum useful temperature of the materials.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

Silicon nitride, proof testing, strength, brittle materials, ceramic turbine,

ceramics.

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